A three-dimensional, six-degree-of-freedom hypersonic vehicle model is being developed that more accurately characterizes vehicle dynamics for control studies. The main focus areas for this paper are the development of the rigid body dynamics equations of motion, the parametrization of the vehicle geometry, the formulation of the three-dimensional aerodynamic loads, and the adaptation of an existing 1D propulsion model. The aerodynamic loads are calculated by using or combining two-dimensional shock/expansion theory and the Taylor-Maccoll equations for conical flow. Comparisons with computational results show good agreement of resultant force and moment for the top and bottom vehicle surfaces and for most trials for the side surfaces. Using these models, the vehicle is trimmed for steady cruise conditions, and its flight dynamics are linearized about that state. This analysis provides information regarding the stability and controllability of a generic hypersonic vehicle in three-dimensional flight.
Accurate and computationally efficient models of unsteady aerodynamic loads are necessary for the development of hypersonic vehicle control algorithms. This work focuses on using convolution of modal step responses to construct a reduced-order model for these loads. In order to allow the model to be valid over a wide range of modal input amplitudes and flight conditions, a nonlinear correction factor is introduced. Not limited to a specific geometry, the correction factor methodology is general enough to be applied to many different two and three-dimensional vehicle configurations. Good correlation is seen between results obtained from the reduced-order model and computational results.
Airbreathing hypersonic cruise vehicles are typically characterized by long, slender bodies with highly coupled engines and airframes. For a case in which the engine is underslung (below the center of gravity), a large elevon control surface is typically necessary to trim the vehicle. The elevon is usually placed at the rear of the vehicle to yield a large moment arm. However, the drawback is that the elevons can cause large perturbations in lift and other undesirable effects. Canard control surfaces are placed on the forebody of the vehicle to counteract these effects as well as aid in low-speed handling. This study looks at how the canards affect the flow over the elevon control surfaces and, in turn, the controllability of the vehicle in general. A two-dimensional analytical formulation is developed and compared with both a series approximation solution and a computational fluid dynamics Euler flowfield solution. The effect of the canard on the elevon, measured using the elevon effectiveness ratio, decreased as the distance between the control surfaces increased. In general, higher Mach numbers combined with higher canard deflection angles resulted in a greater effect on the elevon. Adding a thickness correction, as opposed to assuming that the airfoils were flat plates, actually decreased, on average, the accuracy of the model when compared with the computational data.
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